Carbon-containing modacrylic &amp; aramid bicomponent filament yarns

ABSTRACT

A yarn comprising a plurality of bicomponent filaments having a first region comprising a first polymer composition and a second region comprising a second polymer composition, each of the first and second regions being distinct in the bicomponent filaments; each bicomponent filament comprising 5 to 60 weight percent of the first polymer composition and 95 to 40 weight percent of the second polymer composition; wherein the first polymer composition comprises aramid polymer containing 0.5 to 20 weight percent discrete homogeneously dispersed carbon particles and the second polymer composition comprises modacrylic polymer being free of discrete carbon particles; the yarn having a total content of 0.1 to 5 weight percent discrete carbon particles.

BACKGROUND OF THE INVENTION

Field of the Invention. This invention relates to yarns of bicomponentfilaments suitable for use in arc protection, wherein each filament hasa distinct region of aramid polymer having discrete carbon particleshomogeneously dispersed therein and a distinct region of modacrylicpolymer being free of discrete carbon particles.

Description of Related Art. U.S. Pat. No. 4,803,453 to Hull disclosesmelt-spun filaments having antistatic properties comprising acontinuous, nonconductive sheath of a synthetic thermoplasticfiber-forming polymer surrounding an electrically conductive polymericcore comprised of electrically conductive carbon black dispersed in athermoplastic synthetic polymer.

U.S. Pat. No. 4,309,476 to Nakamura et al. discloses a core-in-sheathtype aromatic polyamide fiber having satisfactory dyeing properties madefrom a single aromatic polyamide material. When the core-in-sheath fiberis dyed with acid dyes, only the sheath portion is colored. U.S. Pat.No. 4,398,995 to Sasaki et al. discloses the use of the fiber ofNakamura in a paper.

U.S. Pat. No. 3,038,239 to Moulds discloses improved composite filamentsthat have crimp reversibility. The filaments have at least twohydrophobic polymers in eccentric relationship, wherein one of thehydrophobic polymers further contains mixed therewith a minor amount ofpolymer having a high water absorption rate.

U.S. Pat. Nos. 7,065,950 and 7,348,059 to Zhu et al. disclose a yarn,fabric, and garment for use in arc and flame protection that containsmodacrylic, p-aramid, and m-aramid fibers. While these fiber blends havebeen found to be very useful in arc protection, any improvement in arcprotection is welcomed as it can potentially save lives.

BRIEF SUMMARY OF THE INVENTION

This invention relates to a yarn comprising a plurality of bicomponentfilaments, the bicomponent filaments having a first region comprising afirst polymer composition and a second region comprising a secondpolymer composition, each of the first and second regions being distinctin the bicomponent filaments; each bicomponent filament comprising 5 to60 weight percent of the first polymer composition and 95 to 40 weightpercent of the second polymer composition; wherein the first polymercomposition comprises aramid polymer containing 0.5 to 20 weight percentdiscrete carbon particles based on the amount of carbon particles in thefirst composition, homogeneously dispersed in the first region in thefilament; and wherein the second polymer composition comprisesmodacrylic polymer being free of discrete carbon particles; the yarnhaving a total content of 0.1 to 5 weight percent discrete carbonparticles.

This invention further relates to a process for forming a yarncomprising bicomponent filaments, each of the filaments comprising adistinct sheath of a modacrylic polymer free of discrete carbonparticles and a distinct core of an aramid polymer comprising discretecarbon particles homogeneously dispersed therein, with the sheathsurrounding the core; the process comprising the steps of:

-   -   a) forming a first polymer solution containing aramid polymer in        a solvent, the aramid polymer solution further comprising        discrete carbon particles, and forming a second polymer solution        of modacrylic polymer being free of discrete carbon particles,        in the same or different solvent;    -   b) providing a spinneret assembly having separate inlets for the        first polymer solution and the second polymer solution and a        plurality of exit capillaries for spinning dope filaments;    -   c) forming a plurality of dope filaments having a sheath of the        second polymer solution and a core of the first polymer solution        by extruding through the exit capillaries a plurality of        conjoined streams of the first and the second solutions into a        spin cell, and    -   d) extracting solvent from the plurality of dope filaments to        make a yarn of polymer filaments.

This invention also relates to a process for forming a yarn comprisingbicomponent filament having side-by-side structure, each of thefilaments comprising a distinct first side of an aramid polymercomprising discrete carbon particles homogeneously dispersed therein anda distinct second side of a modacrylic polymer free of discrete carbonparticles, the process comprising the steps of:

-   -   a) forming a first polymer solution containing aramid polymer in        a solvent, the aramid polymer solution further comprising        discrete carbon particles, and forming a second polymer solution        of modacrylic polymer being free of discrete carbon particles,        in the same or different solvent;    -   b) providing a spinneret assembly having separate inlets for the        first polymer solution and the second polymer solution and a        plurality of exit capillaries for spinning dope filaments;    -   c) forming a plurality of dope filaments having a first side of        the first polymer solution and a second side of the second        polymer solution in a side-by-side orientation by extruding        through the exit capillaries a plurality of conjoined streams of        the first and the second solutions into a spin cell, and    -   d) extracting solvent from the plurality of dope filaments to        make a yarn of polymer filaments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical microscope image of the cross section of thesheath-core bicomponent filaments having a sheath of modacrylic polymerthat is free of carbon particles and a core of poly(metaphenyleneisophthalamide) polymer comprising carbon particles.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to yarns useful in the making of articles thatprovided arc protection for workers and other personnel. An arc flash isan explosive release of energy caused by an electrical arc. Electricalarcs typically involve thousands of volts and thousands of amperes ofelectrical current, exposing the garment to intense incident heat andradiant energy. To offer protection to a wearer, an article ofprotective apparel must resist the transfer of this incident energythrough to the wearer. It has been believed that this occurs best whenthe article of protective apparel absorbs a portion of the incidentenergy while resisting what is called “break-open”. During “break-open”,a hole forms in the article. Therefore, protective articles or garmentsfor arc protection have been designed to avoid or minimize break-open ofany of the fabric layers in the garment.

It has been found that the arc performance of fabrics and garments canbe increased on the order of almost two times by the addition ofdiscrete carbon particles in the polymer of fire-resistant (i.e., havinga limiting oxygen index greater than 21) and thermally stable fiber. Asused herein, the term “thermally stable” means the polymer or fiberretains at least 90 percent of its weight when heated to 425 degreesCelsius at a rate of 10 degrees per minute.

On a fabric weight basis, a dramatic improvement has been found when thetotal amount of discrete carbon particles in the fabric is 0.1 to 3weight percent, based on the total amount of fiber in the fabric. Thepresence of these carbon particles can have a significant effect on thefabric arc performance, as measured by ATPV, even at very low loadings.The best performance is found for carbon particles amounts of greaterthan about 0.5 weight percent in the fabric, with a preferredperformance of 12 cal/cm² or greater occurring for fabrics having about0.75 weight percent carbon particles or greater, with an especiallydesired range being 0.75 to 2 weight percent carbon particles in thefabric.

Specifically, this invention relates to a yarn comprising a plurality ofbicomponent filaments, the bicomponent filaments having a first regioncomprising a first polymer composition and a second region comprising asecond polymer composition; the regions being distinct and preferablyuniform-density in the bicomponent filaments. Preferably the regions arein the form of a sheath-core structure with the first region being thecore and the second region being the sheath. Alternatively, the regionsare in the form of a side-by-side bicomponent structure. The firstpolymer composition comprises aramid polymer containing 0.5 to 20 weightpercent discrete carbon particles based on the amount of carbonparticles in the first composition, homogeneously dispersed in the firstregion in the filament. The second polymer composition comprisesmodacrylic polymer being free of discrete carbon particles. The yarn hasa total content of 0.1 to 5 weight percent discrete carbon particles,based on the amount of carbon particles in all the bicomponent filamentsin the yarn.

Therefore, this invention relates yarns of bicomponent filaments thathave dispersed carbon particles that dramatically improved arcperformance versus a blend of modacrylic and aramid fibers as disclosedin U.S. Pat. Nos. 7,065,950 and 7,348,059. In some embodiments, thefilaments can be further colored in yarn, fabric or article form to helpmask the presence of the black carbon particles in the fiber. In someembodiments the bicomponent filaments further include a spun-in pigmentin the modacrylic polymer sheath to mask the presence of the blackcarbon-containing fiber in the yarn, fabric or article.

The yarn comprises a plurality of bicomponent filaments. “Bicomponent”means the filaments are formed from at least two polymer compositionsthat differ in some way. Since at least two differing polymercompositions are needed in the making of the bicomponent filaments, thismeans that two differing polymer solutions are made; however, the twodiffering polymer solutions can use the same or different solvent.Preferably the solvent is the same for the two differing polymersolutions.

The bicomponent filaments have a first region comprising a first polymercomposition and a second region comprising a second polymer composition.The regions are distinct and preferably uniform-density in a sheath-corestructure or a side-by-side structure. One representative region for thesheath-core structure is the sheath, while another representative regionis the core. Side-by-side structures can have a more oblong or dog-boneshape in cross section, or can be more bean-shaped or round in crosssection, so a representative region is either one side of the filamentor the other. Further, the side-by-side structure can be made whereinthe two sides or regions are similarly sized and substantiallysymmetrical, if the relative amounts of the two polymers are similar; orthe side-by-side structure can be made wherein one side or regionoverlaps the other side or region; that is, one side or region coversmore than 50 percent of the circumference of the other side or region.This can be the case when the relative amounts of the two polymers arevery different, and one side or region can cover 75 percent or more ofthe circumference of the other side or region.

By “distinct” it is meant that the first and second polymer compositionsare appreciably unmixed in the filament, and there is a distinct visibleboundary between the two polymer regions that can be seen by visualinspection under suitable magnification using an optical or electronmicroscope. In the sheath-core structure, preferably the sheath iscontinuous. By “continuous” is meant, in the case of the sheath of thesheath-core filament, that the sheath polymer completely radiallysurrounds the core polymer, and that the coverage of the core polymer bythe polymer sheath is substantially continuous linearly along the lengthof the filament. Preferably the core is continuous or semi-continuous.When referring to the core of the sheath-core filament, by “continuous”is meant the core polymer is substantially continuous linearly along thelength of the filament, and “semi-continuous” means the core may haveminor discontinuities linearly along the filament that do notappreciably affect the ability of the carbon particles in the core tofunction in the filaments as desired. In the side-by-side structure,preferably each of the sides is “continuous”, meaning the polymerregions on each side of the bicomponent filament is substantiallycontinuous linearly along its length. However, in some embodiments, theregion or side containing the carbon particles can be continuous orsemi-continuous, with semi-continuous meaning the carbonparticle-containing region may have minor discontinuities linearly alongthe filament that do not appreciably affect the ability of the carbonparticles in the filament to function in the filaments as desired. Bythe phrase “uniform-density” with regards to the sheath, it is meantthat by visual inspection under suitable magnification using an opticalor electron microscope the filament cross section shows the sheath to begenerally solid and to be free of objectionable porosity. In preferredembodiments, a uniform-density core is also present in the filament. By“uniform-density” with regards to the core, and with regards to each ofthe sides in a side-by-side structure, it is meant that visualinspection under suitable magnification using an optical or electronmicroscope, a majority of the filament cross sections show the filamentsto have solid, dense centers or character and to be relatively free ofobjectionable porosity and voids. In other words, in some preferredembodiments, the core has a substantially solid cross section anduniform density. Further, in some embodiments the sheath-core filamentsare oval, oblong, bean-shaped, cocoon-shaped, dog-bone-shaped, or amixture of these.

There is no requirement that the core be centered in the sheath, or thatthe thickness of the sheath or core be absolutely the same for eachfilament, since each filament can have slight differences in shape dueto the inability to control all forces on the filaments duringformation. However, the relative amount of the polymers or polymersolutions that are used can provide average final dimensions.

The first polymer composition comprises aramid polymer containing 0.5 to20 weight percent discrete carbon particles, and those carbon particlesare homogenously dispersed in the first region of the filament. When thebicomponent structure is sheath core, the first region is the core ofthe filament; when the bicomponent structure is side-by-side, the firstregion is one of the sides of the filament. The phrase “homogeneouslydispersed” means that the carbon particles can be found in the regionuniformly distributed in both the axial and radial directions in thedesired region in the fiber. In some embodiments, the first polymercomposition comprises aramid polymer containing 0.5 to 15 weight percentdiscrete carbon particles; and in some other embodiments the firstpolymer composition comprises aramid polymer containing 0.5 to 10 weightpercent discrete carbon particles. In some embodiments, the firstpolymer composition comprises aramid polymer containing 0.5 to 6 weightpercent discrete carbon particles. In some embodiments it is desirablethe first polymer composition comprises aramid polymer containing 5 to10 weight percent discrete carbon particles. In some embodiments, eachbicomponent filament has a total content of 0.5 to 3 weight percentdiscrete carbon particles, based on the total weight of each filament.

The first polymer composition comprises aramid polymer that preferablyhas a Limiting Oxygen Index (LOI) above the concentration of oxygen inair (that is, greater than 21 and preferably greater than 25). Thismeans the fiber or a fabric made solely from that fiber will not supportflame in the normal oxygen concentrations in air and is consideredfire-resistant. The first polymer further preferably retains at least 90percent of its weight when heated to 425 degrees Celsius at a rate of 10degrees per minute, meaning that this polymer has high thermalstability. It is believed the combination of this fire-resistant andthermally stable polymer and the discrete carbon particlessynergistically provide the improve arc performance.

As present in the fiber, the carbon particles have an average particlesize of 10 micrometers or less, preferably averaging 0.1 to 5micrometers; in some embodiments an average particle size of 0.5 to 3micrometers is preferred. In some embodiments an average particle sizeof 0.1 to 2 micrometers is desirable; and in some embodiments an averageparticle size of 0.5 to 1.5 micrometers is preferred. Carbon particlesinclude such things as carbon black produced by the incompletecombustion of heavy petroleum products and vegetable oils. Carbon blackis a form of paracrystalline carbon that has a highersurface-area-to-volume ratio than soot but lower than that of activatedcarbon. The particles can be incorporated into the fibers by adding thecarbon particles to the spin dope prior to the formation of the fibersvia spinning. In the case of a sheath-core filament, preferably thefirst polymer composition containing carbon particles is present in thecore of the filament.

Essentially any commercially available carbon-black can be used tosupply the discrete carbon particles to the aramid polymer composition.In one preferred practice, a separate stable dispersion of thecarbon-black in a polymer solution, preferably an aramid polymersolution, is first made, and then the dispersion is milled to achieve auniform particle distribution. This dispersion is the preferablyinjected into the aramid polymer solution prior to spinning to form thefirst polymer composition.

The second polymer composition comprises modacrylic polymer, but is freeof discrete carbon particles, meaning that the filament regioncontaining that composition in the does not contain carbon particles asdefined herein.

By modacrylic polymer it is meant preferably the polymer is a copolymercomprising 30 to 70 weight percent of acrylonitrile and 70 to 30 weightpercent of a halogen-containing vinyl monomer. The halogen-containingvinyl monomer is at least one monomer selected, for example, from vinylchloride, vinylidene chloride, vinyl bromide, vinylidene bromide, etc.

In some embodiments the modacrylic copolymers are those of acrylonitrilecombined with vinylidene chloride. In some embodiments, the modacryliccopolymer has in addition antimony oxide or antimony oxides. In somepreferred embodiments the modacrylic copolymer has either less than 1.5weight percent antimony oxide or antimony oxides, or the copolymer istotally free of antimony. Very low antimony content polymer andantimony-free polymer can be made by restricting the amount of, oreliminating entirely, any antimony compounds added to the copolymerduring manufacture. Representative processes for modacrylic polymers,including those that can be modified in this manner are disclosed inU.S. Pat. No. 3,193,602 having 2 weight percent antimony trioxide; U.S.Pat. No. 3,748,302 made with various antimony oxides that are present inan amount of at least 2 weight percent and preferably not greater than 8weight percent; and U.S. Pat. Nos. 5,208,105 & 5,506,042 having 8 to 40weight percent of an antimony compound.

In some embodiments, within the modacrylic polymer has an LOI of atleast 26. In one preferred embodiment the modacrylic polymer has a LOIof at least 26 while also being antimony-free.

In one alternative embodiment, the second polymer composition; that is,the modacrylic polymer composition, further has at least one pigmenthomogeneously dispersed therein to help enable the region in which thatsecond polymer composition is present to preferably mask the present ofthe carbon particles in the other region of the filament. In someembodiments, the at least one masking pigment is present in themodacrylic polymer composition in an amount of 5 to 25 weight percent.In some other embodiments, the at least one masking pigment is presentin the modacrylic polymer composition in an amount of 10 to 20 weightpercent. In some embodiments, the at least one masking pigment ispresent in the bicomponent filaments in an amount of 2.5 to 22.5 weightpercent, based on the total bicomponent filament weight. One especiallypreferred pigment is titanium dioxide (TiO₂).

As used herein, “aramid” is meant a polyamide wherein at least 85% ofthe amide (—CONH—) linkages are attached directly to two aromatic rings.Additives can be used with the aramid and, in fact, it has been foundthat up to as much as 10 percent, by weight, of other polymeric materialcan be blended with the aramid or that copolymers can be used having asmuch as 10 percent of other diamine substituted for the diamine of thearamid or as much as 10 percent of other diacid chloride substituted forthe diacid chloride of the aramid. Suitable aramid fibers are describedin Man-Made Fibers-Science and Technology, Volume 2, Section titledFiber-Forming Aromatic Polyamides, page 297, W. Black et al.,Interscience Publishers, 1968. Aramid fibers are, also, disclosed inU.S. Pat. Nos. 4,172,938; 3,869,429; 3,819,587; 3,673,143; 3,354,127;and 3,094,511.

In some preferred embodiments the aramid polymer is a meta-aramid.Meta-aramid are those aramids where the amide linkages are in themeta-position relative to each other. Preferably the meta-aramid polymerhas an LOI typically at least about 25. One preferred meta-aramid ispoly(metaphenylene isophthalamide).

The bicomponent filaments comprise 5 to 60 weight percent of the firstpolymer composition, which is the aramid polymer composition, and 95 to40 weight percent of the second polymer composition, which is themodacrylic polymer composition. In other words, in the case ofsheath/core filaments, the sheath/core weight ratio ranges from 95:5 to40:60. In some embodiments, the bicomponent filaments comprise 5 to 50weight percent of the first polymer composition and 95 to 50 weightpercent of the second polymer composition, or a sheath/core weight ratiorange of from 95:5 to 50:50. In some embodiments, the bicomponentfilaments comprise 30 to 60 weight percent of the first polymercomposition and 70 to 40 weight percent of the second polymercomposition or a sheath/core weight ratio range of from 70:30 to 40:60.

In one embodiment, this invention further to a process for forming ayarn comprising bicomponent filaments, each of the filaments comprisinga distinct, preferably uniform-density, sheath of a modacrylic polymerfree of discrete carbon particles, and a distinct core of an aramidpolymer comprising discrete carbon particles homogeneously dispersedtherein, with the sheath surrounding the core; the process comprisingthe steps of:

-   -   a) forming a first polymer solution containing the aramid        polymer in a solvent, the aramid polymer solution further        comprising discrete carbon particles, and forming a second        polymer solution containing modacrylic polymer being free of        discrete carbon particles, in the same or different solvent;    -   b) providing a spinneret assembly having separate inlets for the        first polymer solution and the second polymer solution and a        plurality of exit capillaries for spinning dope filaments;    -   c) forming a plurality of dope filaments having a sheath of the        second polymer solution and a core of the first polymer solution        by extruding through the exit capillaries a plurality of        conjoined streams of the first and the second solutions into a        spin cell, and    -   d) extracting solvent from the plurality of dope filaments to        make a yarn of polymer filaments.

If desired, the second polymer solution composition further comprises atleast one masking pigment.

In some embodiments the process is accomplished using dry spinning. Inthis embodiment, the extracting of solvent from the plurality of dopefilaments to make a yarn includes the steps of:

-   -   i) contacting the dope filaments with heated gas in the spin        cell to remove solvent from the dope filaments to form reduced        solvent filaments;    -   ii) quenching the reduced solvent filaments with an aqueous        liquid to cool the filaments, forming a yarn of polymer        filaments; and    -   iii) further extracting solvent from the yarn of polymer        filaments by washing and heating the yarn.

In one embodiment, this invention relates to a process for forming ayarn comprising bicomponent filament having side-by-side structure, eachof the filaments comprising particles a distinct first side of an aramidpolymer comprising discrete carbon particles homogeneously dispersedtherein, and a distinct, preferably uniform-density second side of amodacrylic polymer free of discrete carbon particles, the processcomprising the steps of:

-   -   a) forming a first polymer solution containing aramid polymer in        a solvent, the aramid polymer solution further comprising        discrete carbon particles, and forming a second polymer solution        of modacrylic polymer being free of discrete carbon particles,        in the same or different solvent;    -   b) providing a spinneret assembly having separate inlets for the        first polymer solution and the second polymer solution and a        plurality of exit capillaries for spinning dope filaments;    -   c) forming a plurality of dope filaments having a first side of        the first polymer solution and a second side of the second        polymer solution in a side-by-side orientation by extruding        through the exit capillaries a plurality of conjoined streams of        the first and the second solutions into a spin cell, and    -   d) extracting solvent from the plurality of dope filaments to        make a yarn of polymer filaments.

If desired, the second polymer solution composition further comprises atleast one masking pigment.

In some embodiments the process for forming the side-by-side structureis accomplished using dry spinning. In this embodiment, the extractingof solvent from the plurality of dope filaments to make a yarn includesthe steps of:

-   -   i) contacting the dope filaments with heated gas in the spin        cell to remove solvent from the dope filaments to form reduced        solvent filaments;    -   ii) quenching the reduced solvent filaments with an aqueous        liquid to cool the filaments, forming a yarn of polymer        filaments; and    -   iii) further extracting solvent from the yarn of polymer        filaments by washing and heating the yarn.

In some embodiments, this invention further relates to a process forforming a yarn comprising sheath-core bicomponent filaments having acore comprising carbon particles homogeneously dispersed therein whereinthe yarn further comprises a pigment in the sheath of the bicomponentfilaments for masking the presence of the carbon-containing core,preferably a titanium dioxide pigment. Alternatively, this inventionfurther relates to a process for forming a yarn comprising bicomponentfilaments having a side-by-side structure having a first side comprisingcarbon particles homogeneously dispersed therein wherein the yarnfurther comprises a pigment in the second side of the bicomponentfilaments for masking the presence of the carbon-containing side,preferably a titanium dioxide pigment.

In one embodiment the process includes dry-spinning the yarns ofsheath-core filaments. In general, the term “dry spinning” means aprocess for making filaments by extruding a polymer solution incontinuous streams through spinneret holes into dope filaments into aheated chamber, known as a spin cell that is provided with a heatedgaseous atmosphere. The heated gaseous atmosphere removes a substantialportion of the solvent, generally 40 percent or greater, from the dopefilaments leaving semi-solid filaments having enough physical integritythat they can be further processed. This “dry spinning” is distinct from“wet spinning” or “air-gap wet spinning” (also known as air-gapspinning) wherein the polymer solution is extruded in or directly into aliquid bath or coagulating medium to regenerate the polymer filaments.In other words, in dry spinning a gas is the primary initial solventextraction medium, and in wet spinning (and air-gap wet spinning) aliquid is the primary initial solvent extraction medium. In dryspinning, after sufficient removal of solvent from the dope filamentsand the formation of semi-solid filaments, the filaments can then betreated with additional liquids to cool the filaments and possiblyextract additional solvent from them. Subsequent washing, drawing, andheat treatments can further extract solvent from the filaments in theyarn.

In a preferred embodiment, in the heated spin cell the dope filamentsare contacted or exposed to an environment that contains essentiallyonly inert heated gas and amounts of the solvent removed from the dopefilaments. Preferred inert gases are those that are gases at roomtemperature.

The process involves forming at least two different polymer compositionsin differing solutions, one polymer solution containing modacrylicpolymer in a solvent and being free of carbon particles but optionallyfurther comprising a masking pigment that is not carbon black; andanother polymer solution containing aramid polymer in preferably thesame solvent and containing carbon particles.

The solvent is preferably selected from the group consisting of thosesolvents that also function as proton acceptors, for exampledimethylforamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidone(NMP), and the like. Dimethyl sulfoxide (DMSO) may also be used as asolvent. Dimethylacetamide (DMAc) is one preferred solvent.

The solubility of any particular polymer in any particular solvent isdependent on a variety of parameters, including the relative amounts ofpolymer and solvent, the molecular weight or inherent viscosity of thepolymer, the temperature of the system. Further, while a polymer may beinitially soluble in a solvent, with time, the polymer may precipitateout of the solvent, meaning that the solution is not a stable solution.

In a preferred embodiment, the process uses at least two polymersolutions that are stable polymer spinning solutions. By “stable polymerspinning solution” it is meant that the polymer is soluble in thesolvent or solvent system in a concentration and temperature suitablefor spinning fibers, and that the polymer remains soluble in the solventindefinitely. The term “solvent system” is meant to include a solventand a solubility/stability aid such as an inorganic salt.

In some embodiments, aramid polymer will form a useful stable polymerspinning solution only if a solubilizing/stabilizing salt is present.Therefore, if desired and needed, the aramid polymer solution includesat least 4 percent inorganic salt by weight, based on the amount of thesalt, the polymer, and the solvent in the solution, to maintain thepolymer in solution. In some embodiments the solution includes at least7 weight percent inorganic salt.

Inorganic salts that can be used include chlorides or bromides havingcations selected from the group consisting of calcium, lithium,magnesium or aluminum. Calcium chloride or lithium chloride salts arepreferred. As used herein, the word “salt” is meant to include thecompounds that increase the solubility of the polymer in the selectedsolvent or help provide stable spinning solutions and excludes anyadditives (especially flame retardant additives) that might be salts butare only added to increase the limiting oxygen index of the polymer.Likewise, the term “salt-free” does not mean these LOI-increasingadditives are not present, only that the inorganic salts mentionedpreviously are absent.

In some embodiments, modacrylic polymer will form a useful stablepolymer spinning solution without a solubilizing/stabilizing salt beingpresent. Such solutions are considered salt-free and are preferred.

Useful polymer solutions are those that can be extruded, preferablydry-spun, into useful dope filaments. Parameters that can be balanced toform useful polymer solutions include the polymer molecular weight andconcentration of the polymer in the solvent. Obviously, the specificparameters are dependent on the polymer and solvent chosen. However, itis known that certain polymer solutions of a certain viscosity tend tomake useful filaments. All of the variables that could impact viscosity,e.g., temperature, concentration, polymer and solvent type, polymermolecular weight, etc. can be used to create a useful polymer solution.Generally, such solutions have a so-called zero shear rate or Newtonianviscosity of about 10 to 1000 Pascal seconds (Pa-sec) and preferablyabout 50 to 500 Pa-sec.

After forming at least the first and the second compositions andsolutions, the dry spinning process includes providing a spinneretassembly having separate inlets for the first solution and the secondsolution, and a plurality of exit capillaries for extruding (spinning)dope filaments. One preferred spinneret assembly useful for spinning thedope filaments is disclosed in U.S. Pat. No. 5,505,889 to Davies.However, other spinneret assemblies are potentially useful and can havemany different features such as the spinneret assemblies shown in U.S.Pat. Nos. 2,936,482; and 3,541,198, which are just some of the possiblespinneret assemblies that can be used.

The process can further involve forming a plurality of dope filamentshaving preferably a continuous sheath of the modacrylic polymer that isfree of carbon particles and a continuous core of aramid polymer thatcontains carbon particles. The core need not be strictly continuous toprovide adequate carbon particles for the bicomponent filaments toperform as desired. Alternatively, the plurality of dope filamentshaving a continuous region of the modacrylic polymer that is free ofcarbon particles is spun together with a continuous region of aramidpolymer that contains carbon particles in a side-by-side bicomponentfilament structure. Both of these filament structures are made byextruding through the exit capillaries in the spinneret assembly aplurality of conjoined streams of the first and second solutions into aspin cell. For the purposes herein, “spin cell” is meant to include anysort of chamber or bath that can remove solvent from the dope filaments.

In a preferred embodiment, the first solution and the second solutionare supplied via separate inlets to and into the spinneret assemblywhere they are combined. In some embodiments the spinneret assemblydistributes the two solutions such that the two solutions are bothsupplied to each exit capillary in the spinneret assembly, which forms abicomponent dope filament preferably having a continuous sheath of themodacrylic polymer solution and a semi-continuous or continuous core ofan aramid polymer solution, made by conjoining the first and secondpolymer solutions in each exit capillary of the spinneret. That is, thesolutions are supplied in a manner suitable to provide a sheath-corefilament structure and then extruded through the same exit capillary,each exit capillary being one of a plurality of exit capillaries in thespinneret assembly. While this is a preferred embodiment, any otherarrangement of exit capillaries or apertures or methods that conjoinsthe first and second polymer solutions into suitable bicomponent dopefilaments of the desired structures, such as the alternativeside-by-side structure, can be used.

The preferred process continues with contacting the dope filaments withheated gas in the spin cell to remove solvent from the plurality of dopefilaments to form reduced solvent filaments. The heated gas is generallyan inert gas like nitrogen. In some embodiments the dope filaments arecontacted solely with the heated gas in the spin cell.

In some embodiments, of the total solvent in the plurality of dopefilaments, as much as 50 to 85 percent of that solvent is removed fromthe dope filaments in the spin cell. The dope filaments are thereforeconverted to reduced-solvent filaments in the spin cell. Thereduced-solvent filaments are then quenched with an aqueous liquid tocool the filaments, forming a yarn of polymer filaments. The quench alsoserves to remove some of the surface tackiness from the filaments formore efficient downstream processing. Further, the quench can removesome additional solvent, and once quenched it is possible that 75percent or higher of the total original solvent in the dope filamentshas been removed. Additional steps to further extract solvent from theyarn of polymer filaments are then conducted. These steps can includeadditional washing, drawing, and/or heating or heat treating, asdesired.

By “yarn” is meant an assemblage of fibers spun or twisted together toform a continuous strand. As used herein, a yarn generally refers to theassemblage of bicomponent filaments that are spun which are known as acontinuous multifilament yarn. However, the filaments spun herein can becut into staple fiber and converted into what is known in the art as asingles yarn, which is the simplest strand of textile material suitablefor such operations as weaving and knitting. For example, a staple fiberyarn can be formed from the bicomponent fibers in staple fiber form, theyarn having more or less twist. When twist is present in a singles yarn,it is all in the same direction. As used herein the phrases “ply yarn”and “plied yarn” can be used interchangeably and refer to two or moreyarns, i.e. singles yarns, twisted or plied together.

For purposes herein, the terms “fiber” and “filament” are usedinterchangeably and are defined as a relatively flexible,macroscopically homogeneous body having a high ratio of length to thewidth of the cross-sectional area perpendicular to that length. Also,such fibers preferably have a generally solid cross section for adequatestrength in textile uses; that is, the fibers preferably are notappreciably voided or do not have a large quantity of objectionablevoids.

If desired, the yarns can comprise the herein described bicomponentfibers that are blended with other fibers, in either continuousmultifilament or staple form. Also, the yarns of bicomponent filamentscan be cut into staple fibers. As used herein, the term “staple fibers”refers to fibers that are cut to a desired length or are stretch broken,or fibers that are made having a low ratio of length to the width of thecross-sectional area perpendicular to that length, when compared withcontinuous filaments. Man-made staple fibers are cut or made to a lengthsuitable for processing on, for example, cotton, woolen, or worsted yarnspinning equipment. The staple fibers can have (a) substantially uniformlength, (b) variable or random length, or (c) subsets of the staplefibers have substantially uniform length and the staple fibers in theother subsets have different lengths, with the staple fibers in thesubsets mixed together forming a substantially uniform distribution.

In some embodiments, suitable staple fibers have a cut length of from 1to 30 centimeters (0.39 to 12 inches). In some embodiments, suitablestaple fibers have a length of 2.5 to 20 cm (1 to 8 in). In somepreferred embodiments the staple fibers made by short staple processeshave a cut length of 6 cm (2.4 in) or less. In some preferredembodiments the staple fibers made by short staple processes have astaple fiber length of 1.9 to 5.7 cm (0.75 to 2.25 in) with the fiberlengths of 3.8 to 5.1 cm (1.5 to 2.0 in) being especially preferred. Forlong staple, worsted, or woolen system spinning, fibers having a lengthof up to 16.5 cm (6.5 in) are preferred.

The staple fibers can be made by any process. For example, the staplefibers can be cut from continuous straight fibers using a rotary cutteror a guillotine cutter resulting in straight (i.e., non-crimped) staplefiber, or additionally cut from crimped continuous fibers having a sawtooth shaped crimp along the length of the staple fiber, with a crimp(or repeating bend) frequency of preferably no more than 8 crimps percentimeter. Preferably the staple fibers have crimp.

The staple fibers can also be formed by stretch breaking continuousfibers resulting in staple fibers with deformed sections that act ascrimps. Stretch broken staple fibers can be made by breaking a tow or abundle of continuous filaments during a stretch break operation havingone or more break zones that are a prescribed distance creating a randomvariable mass of fibers having an average cut length controlled by breakzone adjustment.

Spun staple yarn can be made from staple fibers using traditional longand short staple ring spinning processes that are well known in the art.However, this is not intended to be limiting to ring spinning becausethe yarns may also be spun using air jet spinning, open end spinning,and many other types of spinning that converts staple fiber into useableyarns. Spun staple yarns can also be made directly by stretch breakingusing stretch-broken tow-to-top staple processes. The staple fibers inthe yarns formed by traditional stretch break processes typically havelength of up to 18 cm (7 in) long; however, spun staple yarns made bystretch breaking can also have staple fibers having maximum lengths ofup to around 50 cm (20 in.) through processes as described for examplein PCT Patent Application No. WO 0077283. Stretch broken staple fibersnormally do not require crimp because the stretch-breaking processimparts a degree of crimp into the fiber.

The staple fiber made from the bicomponent filaments, or the bicomponentfilaments themselves can further be used in a fiber blend if desired. Byfiber blend it is meant the combination of two or more staple fibertypes, or two or more continuous filaments, in any manner. Preferablythe staple fiber blend is an “intimate blend”, meaning the variousstaple fibers in the blend form a relatively uniform mixture of thefibers. In some embodiments the two or more staple fiber types areblended prior to or while the staple fiber yarn is being spun so thatthe various staple fibers are distributed homogeneously in the stapleyarn bundle.

The blend optionally contains antistat fibers. One suitable fiber is amelt-spun thermoplastic antistatic fiber in an amount of 1 to 3 weightpercent, such as those described in U.S. Pat. No. 4,612,150 to De Howittand/or U.S. Pat. NO. 3,803,453 to Hull. These fibers, while they containcarbon black, have a negligible impact on arc performance, since thefiber polymer does not have the combination of being flame resistant andthermally stable; that is, it does not have in combination a LOI ofgreater than 21 and does not retain at least 90 percent of its weightwhen heated to 425 degrees Celsius at a rate of 10 degrees per minute.In fact, such thermoplastic antistat fibers lose in excess of 35 weightpercent when heated to 425 degrees Celsius at a rate of 10 degrees perminute. For the purposes herein, and to avoid any confusion, the totalcontent in the weight percent of discrete carbon particles is based onthe total weight of the fiber blend, excluding any minor amount ofantistat fibers.

Fabrics can be made from the yarns, and in some embodiments thepreferred fabrics can include, but are not limited to, woven or knittedfabrics. General fabric designs and constructions are well known tothose skilled in the art. By “woven” fabric is meant a fabric usuallyformed on a loom by interlacing warp or lengthwise yarns and filling orcrosswise yarns with each other to generate any fabric weave, such asplain weave, crowfoot weave, basket weave, satin weave, twill weave, andthe like. Plain and twill weaves are believed to be the most commonweaves used in the trade and are preferred in many embodiments.

By “knitted” fabric is meant a fabric usually formed by interloopingyarn loops by the use of needles. In many instances, to make a knittedfabric, spun staple yarn is fed to a knitting machine which converts theyarn to fabric. If desired, multiple ends or yarns can be supplied tothe knitting machine either plied of unplied; that is, a bundle of yarnsor a bundle of plied yarns can be co-fed to the knitting machine andknitted into a fabric, or directly into an article of apparel such as aglove, using conventional techniques. The tightness of the knit can beadjusted to meet any specific need. A very effective combination ofproperties for protective apparel has been found in for example, singlejersey knit and terry knit patterns.

In some particularly useful embodiments, the spun staple yarns can beused to make arc-resistant and flame-resistant garments. In someembodiments the garments can have essentially one layer of theprotective fabric made from the spun staple yarn. Garments of this typeinclude jumpsuits, coveralls, pants, shirts, gloves, sleeves and thelike that can be worn in situations such as chemical processingindustries or industrial or electrical utilities where an extremethermal event might occur. In one preferred embodiment, the garment ismade from the fabric comprising the yarns described herein.

Protective articles or garments of this type include protective coats,jackets, jumpsuits, coveralls, hoods, etc. used by industrial personnelsuch as electricians and process control specialists and others that maywork in an electrical arc potential environment. In a preferredembodiment, the protective garment is a coat or jacket, including athree-quarter length coat commonly used over the clothes and otherprotective gear when work on an electrical panel or substation isrequired.

In a preferred embodiment, the protective articles or garments in asingle fabric layer have a ATPV of greater than 2 cal/cm²/oz, which isat least a Category 1 or 2 arc rating or higher as measured by either oftwo common category rating systems for arc ratings. The National FireProtection Association (NFPA) has 4 different categories with Category 1having the lowest performance and Category 4 having the highestperformance. Under the NFPA 70E system, Categories 1, 2, 3, and 4correspond to a minimum threshold heat flux through the fabric of 4, 8,25, and 40 calories per square centimeter, respectively. The NationalElectric Safety Code (NESC) also has a rating system with 3 differentcategories with Category 1 having the lowest performance and Category 3having the highest performance. Under the NESC system, Categories 1, 2,and 3 correspond to a minimum threshold heat flux through the fabric of4, 8, and 12 calories per square centimeter, respectively. Therefore, afabric or garment having a Category 2 arc rating can withstand a thermalflux of 8 calories per square centimeter, as measured per standard setmethod ASTM F1959 or NFPA 70E.

In some embodiments, the fabics and articles preferably have an “L*”value ranging from 30 to 70.

Test Methods

Arc Resistance. The arc resistance of fabrics of this invention isdetermined in accordance with ASTM F-1959-99 “Standard Test Method forDetermining the Arc Thermal Performance Value of Materials forClothing”. Preferably fabrics of this invention have an arc resistance(ATPV) of at least 0.8 calories and more preferably at least 2 caloriesper square centimeter per ounce per square yard.

ThermoGravimetric Analysis (TGA). Fiber that retains at least 90 percentof its weight when heated to 425 degrees Celsius at a rate of 10 degreesper minute can be determined using a Model 2950 ThermogravimetricAnalyzer (TGA) available from TA Instruments (a division of WatersCorporation) of Newark, Del. The TGA gives a scan of sample weight lossversus increasing temperature. Using the TA Universal Analysis program,percent weight loss can be measured at any recorded temperature. Theprogram profile consists of equilibrating the sample at 50 degrees C.;ramping the temperature 10° C. per minute from 50 to 1000 degrees C.;using air as the gas, supplied at 10 ml/minute; and using a 500microliter ceramic cup (PN 952018.910) sample container. A specifictesting procedure is as follows. The TGA was programmed using the TGAscreen on the TA Systems 2900 Controller. The sample ID was entered andthe planned temperature ramp program of 20 degrees per minute selected.The empty sample cup was tared using the tare function of theinstrument. The fiber sample was cut into approximately 1/16″ (0.16 cm)lengths and the sample pan was loosely filled with the sample. Thesample weight should be in the range of 10 to 50 mg. The TGA has abalance therefore the exact weight does not have to be determinedbeforehand. None of the sample should be outside the pan. The filledsample pan was loaded onto the balance wire making sure the thermocoupleis close to the top edge of the pan but not touching it. The furnace israised over the pan and the TGA is started. Once the program iscomplete, the TGA will automatically lower the furnace, remove thesample pan, and go into a cool down mode. The TA Systems 2900 UniversalAnalysis program is then used to analyze and produce the TGA scan forpercent weight loss over the range of temperatures.

Limited Oxygen Index. The limited oxygen index (LOI) of fabrics of thisinvention is determined in accordance with ASTM G-125-00 “Standard TestMethod for Measuring Liquid and Solid Material Fire Limits in GaseousOxidants”.

Color Measurement. The system used for measuring color and spectralreflectance is the 1976 CIELAB color scale (L*-a*-b* system developed bythe Commission Internationale de I'Eclairage). In the CIE “L*-a*-b*”system, color is viewed as point in three-dimensional space. The “L*”value is the lightness coordinate with high values being the lightest,the “a*” value is the red/green coordinate with “+a*” indicating red hueand “−a*” indicating green hue and the “b*” value is the yellow/bluecoordinate with “+b*” indicating yellow hue and “−b*” indicating bluehue. A spectrophotometer was used to measure the color of samples,either in puffs of fiber or in fabric or garment form as indicated.Specifically, a Hunter Lab UltraScan® PRO spectrophotometer was used,including the industry standard of 10-degree observer and D65illuminant. The color scale used herein uses the coordinates of the CIE(“L*-a*-b*) color scale with the asterisk, as opposed to the coordinatesof the older Hunter color scale, which are designated (“L-a-b”) withoutthe asterisk.

Weight Percent of Carbon Particles. The nominal amount of carbon blackin the fiber, when making the fiber, is determined by a simple massbalance of ingredients. After the fiber is made, the amount of carbonblack present in the fiber can be determined by measuring the weight ofa sample of fiber, removing the fiber by dissolution of the polymer in asuitable solvent that does not affect the carbon black particles,washing the remaining solids to remove any inorganic salts that are notcarbon, and weighing the remaining solids. One specific method includesweighing about a gram of the fiber, yarn, or fabric to be tested andheating that sample in an oven at 105° C. for 60 minutes to remove anymoisture, followed by placing the sample in a desiccator to cool to roomtemperature, followed by weighing the sample to obtain an initial weightto a precision of 0.0001 grams. The sample is then placed in a 250 mlflat bottom flask with a stirrer and 150 ml of a suitable solvent, forexample 96% sulfuric acid, is added. The flask is then placed on acombination stir/heater with a chilled water condenser operating withenough flow to prevent any fumes from exiting the top of the condenser.The heat is then applied while stirring until the yarn is fullydissolved in the solvent. The flask is then removed from the heater andallowed to cool to room temperature. The contents of the flask are thenvacuum filtered using a Millipore vacuum filter unit with a tared 0.2micron PTFE filter paper. Remove the vacuum and then rinse the flask outwith 25 ml of additional solvent, which is also passed through thefilter. The Millipore unit is then removed from the vacuum flask andreset on a new clean glass vacuum flask. With vacuum, the residue on thefilter paper is washed with water until a pH paper check on the filtrateindicates the wash water to be neutral. The residue is then finallywashed with methanol. The filter paper with residue sample is removed,placed in a dish, and heated in an oven at 105° C. to dry for 20minutes. The filter paper with residue sample in then put in adesiccator to cool to room temperature, followed by weighing the filterpaper with residue sample to obtain the final weight to a precision of0.0001 grams. The weight of the filter is subtracted from the weight ofthe filter paper with residue sample. This weight is then divided by theinitial weight of the yarn or fiber or fabric and multiplied by 100.This will give the weight percentage of the carbon black in the fiber,yarn, or fabric.

Particle Size. Carbon particle size can be measured using the generalprovisions of ASTM B822-10—“Standard Test Method for Particle SizeDistribution of Metal Powders and Related Compounds by LightScattering”.

Weight Percent of Pigments. The nominal amount of pigment in the fiberthat is not carbon black, when making the fiber, is determined by asimple mass balance of ingredients. After the fiber is made, the amountof pigment present in the fiber can be determined by a general method ofmeasuring the weight of a sample of fiber, ashing the sample, andweighing the remaining solids to calculate a weight percent. Onespecific method for determining the amount of TiO₂ in a fiber sampleincludes weighing about 5 grams of the fiber to be tested and heatingthat sample in an oven at 105° C. for 60 minutes to remove any moisture,followed by placing the sample in a desiccator for about 15 minutes tocool to room temperature. A synthetic quartz crucible is then placed ina muffle furnace operating at 800° C. for 15 minutes, after which it isremoved and allowed to cool in a desiccator for 15 minutes. The crucibleis then weighed to a precision of 0.0001 grams. The dried yarn sample isalso weighed to a precision of 0.0001 grams to obtain its initialweight. The dried yarn sample is placed in the crucible, and thecrucible with sample is then placed in the muffle furnace operating at800° C. for 60 minutes. The crucible is then removed and is placed in adesiccator to cool for 15 minutes, after which the final sample pluscrucible is weighed to a precision of 0.0001 grams. The amount of TiO₂is then calculated by first subtracting the weight of the crucible fromthe weight of the final sample plus crucible, and then dividing thatamount by the initial weight of the fiber sample, followed bymultiplying by 100. This provides the amount of TiO₂ in weight percent.

Shrinkage. To test for fiber shrinkage at elevated temperatures, the twoends of a sample of multi-filament yarn to be tested are tied togetherwith a tight knot such that the total interior length of the loop isapproximately 1 meter in length. The loop is then tensioned until tautand the doubled length of the loop measured to the nearest 0.1 cm. Theloop of yarn is then hung in an oven for 30 minutes at 185 degreesCelsius. The loop of yarn is then allowed to cool, it is re-tensionedand the doubled length is re-measured. Percent shrinkage is thencalculated from the change in the linear length of the loop.

EXAMPLE 1

In this example, a sheath/core bicomponent fiber was spun with a 40%sheath of modacrylic polymer and a 60% core with poly (metaphenyleneisophthalamide) (MPD-I) with a 2.5 weight percent carbon-black loadingin the core without any pigment in the sheath (1.5 weight percent carbonblack loading in the fiber).

A 30 weight percent solution of modacrylic polymer in DMAc was preparedby dissolving a suitable amount of fiber made from modacrylic polymer inDMAc. A 19.3 weight percent solution of MPD-I polymer in DMAc was alsoobtained by polymerizing the polymer in that solvent.

Independently controlled streams of the modacrylic polymer solution andMPD-I polymer solution were then fed to spin-cell for dry spinning asheath of modacrylic polymer over a core of MPD-I polymer. The spin-cellwas equipped with a spinneret assembly to produce the sheath/corefilaments before entering into the top of the spin-cell. The spinneretassembly, such as detailed in FIGS. 1-3 of U.S. Pat. No. 5,505,889,included a meter plate and spinneret to produce the desired sheath-corestructure from the two solutions. The spinneret was provided with 791holes, each with a diameter of 0.005 inches and a length of 0.01 inches.The spinneret assembly further comprised of steam passages such that thetemperature of the solutions as they traveled through the meter plateand spinneret are controlled between 100 and 150° C.

The flow rate of the modacrylic and MPD-I polymer solutions in thestreams was controlled independently with two different metering pumps.A separate additive dispersion containing carbon-black was made bymixing an amount of carbon-black with a MPD-I solution and then millingthe dispersion to achieve a uniform particle distribution. Thisdispersion was then injected into the core stream of MPD-I polymersolution prior to the spinneret at a flow rate suitable to obtain a 2.5weight percent carbon-black loading in this stream.

The bundle of individual modacrylic/MPD-I sheath/core filaments withcarbon-black leaving the spinneret assembly were subjected to heatednitrogen gas to remove some of the DMAc from the filaments; thefilaments were further quenched with an aqueous liquid at the exit ofthe spin-cell.

A sample of the threadline bundle produced from this process was takenafter quenching and an optical microscope image of the cross section ofthe sheath-core bicomponent filaments was taken and shown in FIG. 1. Thesheath core structure of the fiber is apparent along with the blackcore.

The quenched modacrylic/MPD-I sheath/core fiber with carbon-black wasfurther processed on a wash/draw machine to further draw the filaments3.6 times and extract additional DMAc solvent such that the residualconcentration was less than 1 weight percent. The fiber was then driedand further heat treated using steam dryers and electrically heateddrums, followed by the application of spin-finish before being wound ona bobbin.

EXAMPLE 2

Examples 1 is repeated but with a spinneret that provides a side-by-sidebicomponent filament structure, wherein the weight ratio of the firstside of aramid polymer containing carbon particles to the second side ofmodacrylic polymer without carbon particles is 60:40 as in that example.Due to the additional mass on the first side, that side encloses morethan 50 percent of the circumference of the second side, making aside-by-side bicomponent filament that functions similar to asheath-core.

EXAMPLE 3

The procedure shown in Examples 1 and 2 is repeated, however, inaddition the modacrylic stream contains a dispersion of rutile titaniumdioxide (TiO₂) pigment in DMAc. This stable dispersion contains polymerand about 30 weight percent TiO₂ in DMAc, which is milled to achieve auniform distribution of the TiO₂ in the dispersion. This milleddispersion is then added to the stream of modacrylic polymer in anamount that is equal to a final concentration of 15 weight percent TiO₂in the modacrylic polymer. In this matter sheath-core filaments havingthe TiO₂ pigment in the sheath, and side-by-side filaments having theTiO₂ pigment in one side, are made.

What is claimed is:
 1. A yarn comprising a plurality of bicomponentfilaments, the bicomponent filaments having a first region comprising afirst polymer composition and a second region comprising a secondpolymer composition, each of the first and second regions being distinctin the bicomponent filaments; each bicomponent filament comprising 5 to60 weight percent of the first polymer composition and 95 to 40 weightpercent of the second polymer composition; wherein the first polymercomposition comprises aramid polymer containing 0.5 to 20 weight percentdiscrete carbon particles based on the amount of carbon particles in thefirst composition, homogeneously dispersed in the first region in thefilament; and wherein the second polymer composition comprisesmodacrylic polymer being free of discrete carbon particles; the yarnhaving a total content of 0.1 to 5 weight percent discrete carbonparticles.
 2. The yarn of claim 1 wherein each bicomponent filamentcomprises 30 to 60 weight percent of the first polymer composition and70 to 40 weight percent of the second polymer composition.
 3. The yarnof claim 1 wherein the first polymer composition comprises aramidpolymer containing 0.5 to 10 weight percent discrete carbon particles.4. The yarn of claim 1 wherein the second polymer composition furthercomprises at least one masking pigment.
 5. The yarn of claim 4 whereinthe at least one masking pigment is present in the second polymercomposition in an amount of 5 to 25 weight percent.
 6. The yarn of claim5 wherein the at least one masking pigment is present in the secondpolymer composition in an amount of 10 to 20 weight percent.
 7. The yarnof claim 1 wherein the at least one masking pigment is present in thebicomponent filaments in an amount of 2.5 to 22.5 weight percent.
 8. Theyarn of claim 1 wherein the bicomponent filaments have a sheath-corestructure with the first region being the core and the second regionbeing the sheath.
 9. The yarn of claim 1 wherein the bicomponentfilaments have a side-by-side structure with the first region being afirst side of the filament and the second region being a second side ofthe filament.
 10. The yarn of claim 1 wherein the yarn has a totalcontent of 0.1 to 3 weight percent discrete carbon particles
 11. Afabric comprising the yarn of claim
 1. 12. An article of thermalprotective clothing comprising the fabric of claim
 11. 13. An article ofthermal protective clothing comprising the yarn of claim
 1. 14. Aprocess for forming a yarn comprising bicomponent filaments, each of thefilaments comprising a distinct sheath of a modacrylic polymer free ofdiscrete carbon particles and a distinct core of an aramid polymercomprising discrete carbon particles homogeneously dispersed therein,with the sheath surrounding the core; the process comprising the stepsof: a) forming a first polymer solution containing aramid polymer in asolvent, the aramid polymer solution further comprising discrete carbonparticles, and forming a second polymer solution of modacrylic polymerbeing free of carbon particles in the same or different solvent; b)providing a spinneret assembly having separate inlets for the firstpolymer solution and the second polymer solution and a plurality of exitcapillaries for spinning dope filaments; c) forming a plurality of dopefilaments having a sheath of the second polymer solution and a core ofthe first polymer solution by extruding through the exit capillaries aplurality of conjoined streams of the first and the second solutionsinto a spin cell, and d) extracting solvent from the plurality of dopefilaments to make a yarn of polymer filaments.
 15. The process of claim14 wherein the step d) of extracting solvent from the plurality of dopefilaments to make a yarn includes the steps of: i) contacting the dopefilaments with heated gas in the spin cell to remove solvent from thedope filaments to form reduced solvent filaments; ii) quenching thereduced solvent filaments with an aqueous liquid to cool the filaments,forming a yarn of polymer filaments; and iii) further extracting solventfrom the yarn of polymer filaments by washing and heating the yarn. 16.The process of claim 14 wherein the second polymer solution compositionfurther comprises at least one masking pigment.
 17. The process of claim14 wherein the aramid polymer is poly(metaphenylene isophthalamide). 18.A process for forming a yarn comprising bicomponent filament havingside-by-side structure, each of the filaments comprising particles adistinct first side of an aramid polymer comprising discrete carbonparticles homogeneously dispersed therein and a distinct second side ofa modacrylic polymer free of discrete carbon particles; the processcomprising the steps of: a) forming a first polymer solution containingaramid polymer in a solvent, the aramid polymer solution furthercomprising discrete carbon particles, and forming a second polymersolution of modacrylic polymer being free of discrete carbon particles,in the same or different solvent; b) providing a spinneret assemblyhaving separate inlets for the first polymer solution and the secondpolymer solution and a plurality of exit capillaries for spinning dopefilaments; c) forming a plurality of dope filaments having a first sideof the first polymer solution and a second side of the second polymersolution in a side-by-side orientation by extruding through the exitcapillaries a plurality of conjoined streams of the first and the secondsolutions into a spin cell, and d) extracting solvent from the pluralityof dope filaments to make a yarn of polymer filaments.
 19. The processof claim 18 wherein the step d) of extracting solvent from the pluralityof dope filaments to make a yarn includes the steps of: i) contactingthe dope filaments with heated gas in the spin cell to remove solventfrom the dope filaments to form reduced solvent filaments; ii) quenchingthe reduced solvent filaments with an aqueous liquid to cool thefilaments, forming a yarn of polymer filaments; and iii) furtherextracting solvent from the yarn of polymer filaments by washing andheating the yarn.
 20. The process of claim 18 wherein the second polymersolution composition further comprises at least one masking pigment. 21.The process of claim 18 wherein the aramid polymer is poly(metaphenyleneisophthalamide).